The subject matter of the present invention is a novel device and a novel method that permit the determination at high spatial resolution of the spatial structure of a sample to be examined. A plurality of different material characteristics can simultaneously be investigated, which are substantially merely limited by the specific basic method of microscopy chosen. In principle the method according to the invention allows spatial resolutions in the atomic range (100 picometer).
Meanwhile, a plurality of methods have become available in microscopy. The most widely known methods are methods of optical microscopy, the resolving power of which is on principle limited to a range of some hundred nanometers. Considerably higher resolving power is attained in electron microscopy that exists in various types. A resolution within the range of some nanometers can be routinely obtained by means of electron microscopy. In the course of the last 15 years, further methods of microscopy were added of which the most outstanding are the methods of so-called scanning probe microscopy which permit atomic resolution in using specific probes and which may be sensitive to a plurality of material properties. Sharp metallic tips, sharp tips on a cantilever (a thin beam to be curved by external forces) or stretched tips of optical waveguides may serve as probes for example. These probes are generally run at a very close distance (some nanometers and less) across the sample (the sample is scanned) and certain material parameters are investigated at the same time. Electric currents between the surface of the sample and the tip, a mechanical distortion or torsion of the cantilever, the number of protons intercepted by the waveguide or even the attenuation of mechanical oscillations of the probe for example may serve as control parameters for controlling the spacing between the sample and the tip.
All these methods of microscopy, which have only been mentioned by way of example, have in common that they only allow the production of two-dimensional images of the objects to be examined. For anyone interested in the spatial structure of samples to be examined, the number of available methods of investigation is strongly limited.
A greater number of methods of determining structures do not investigate the spatial structure of the samples directly but in a roundabout way by making use of so-called scattering techniques (e.g. X ray scattering or neutron scattering). These scattering methods, however, are to be essentially used on objects that have a regular structure. Accordingly, scattering methods are particularly suited to determine crystal structures, but they are not to be used, or only with severe limitations, on samples having an irregular structure.
The majority of the methods that permit one to determine at high spatial resolution the spatial structure of an irregular sample is based on the fact that the volume of the sample is divided into a sequence of layers, a map of the properties of interest being built for each layer. Then, the spatial distribution of the properties of interest is reconstructed from a heap of such maps by means of appropriate mathematical methods. Various methods operate on this principle and differ in the manner of imaging the investigated properties (the contrast method) and in the way of dividing the volume of the sample in individual layers and of building the map of the properties of interest for each layer.
The best known methods to the present day are the X ray tomography and the magnetic resonance imaging technique. In both methods, but one single thin layer of the volume of the sample is nondestructively detected by an appropriate measuring technique and a map of the properties of interest is built in this layer from a large number of projections of the layer taken from various angles by means of mathematical methods. Both methods permit one to attain a spatial resolution of some micrometers. Confocal microscopy falls into this same class and permits one to build up an image of the focal plane with very little depth of definition. However, in confocal microscopy, the spatial resolution is limited to some hundred nanometers by the wavelength of the utilized light and optically transparent specimens only can be examined.
In the historically first methods of spatial reconstruction, the sample was mechanically divided into a series of individual thin layers by means of a microtome. A sequence of maps was then built from these thin sections using optical microscopy, which permitted one to reconstruct the spatial shape of the investigated objects. Today, some thin layers, of some nanometers thin, may be produced by means of an ultramicrotome and may be studied with the scanning electron microscope at a high lateral resolution (in the x-y-plane) (several nanometers are attainable). Resolution in depth (in z direction) of this method however is limited by the thickness of the thin sections employed. Said thickness is in turn limited to some several nanometers by the mechanical stability of the investigated material. A major drawback to this method is the high need for time and staff since reliable manufacturing, manipulation and investigation of the extremely thin serial sections require much experience and occur in great parts manually. Additionally, sections thus thin cannot be made from all the materials so that metals, ceramics, semiconductors and many other significant raw materials cannot be studied by using this method.
The X ray tomography and the magnetic resonance imaging technique extensively use mathematical methods to reconstruct the distribution of the property investigated in one single examined layer from a large number of projections taken from various angles. Similar methods may also be employed in electron microscopy. In particular cases, a resolution in the range of nanometers is attainable. In the thin sections, of some several nanometers thick, which are required for this purpose, the maximum time of action of the electrons that one single thin section is able to bear before it is destroyed by the beam of electrons is limited, the limited overall time of action of the electrons having to be distributed over the individual projections as a result thereof, which entails severe limitation of the image quality of the individual projections. For this reason, for the purpose of reconstructing the image, model assumptions on the symmetry of the sample must be made in order to be capable of attaining a resolution ranging in nanometers. Therefore, irregular structures in the sample cannot be determined with such a high spatial resolution.
The problem of making the sections and of the mechanical stability of the thin sections is circumvented by methods in which the sample is ablated layer by layer, the distribution of the properties of interest being determined after each ablation on the bare surface. The technique of the dynamic secondary-ion mass spectroscopy (SIMS) operates according to this method. The lateral resolution (in the x-y-plane) is, however, limited by the diameter of the beam of ions that generates the secondary ions and that, at best, is approximately 50 nm large. In the dynamic SIMS, the attainable resolution in depth (in z-direction) is limited by the depth of penetration of the beam of ions into the material (approximately 10 nm and more) and by the roughness of the sample surface prior to and more particularly during ablation. In many cases and specifically with heterogeneous samples, the material is unevenly ablated since the rate of ablation is a property that depends on the material, which causes the surface of the sample to become rough in the course of the investigation. As a result, the detected property (e.g. the concentration of a certain element) originates concurrently from various depths which drastically impairs resolution in depth of the dynamic SIMS. For this reason, many interesting (since heterogeneous) samples cannot be examined using this method, since no procedure of even ablation that suits the dynamic SIMS is known for such samples. The problem of the roughness of the sample and the irregular rate of ablation also prevents the structure of heterogeneous samples from being examined at a high spatial resolution by combining the technique of ablating layer by layer with other methods of high lateral resolution such as, e.g. the scanning electron microscope.
All the methods mentioned have in common that the volume of the sample is divided into a sequence of plane layers and that a plane 2-dimensional map of the property of interest is built for each layer. In the method of the serial sections, resolution in depth is limited by the thickness of the thin sections and amounts to some nanometers. Moreover, many materials are not suited for producing serial sections. In the methods of ablating layers, resolution in depth is in many cases severely limited by the irregular rate of ablation and by the thus occasioned roughness of the surface of the sample.
The object of the present invention is a novel device for determining the spatial distribution of properties of a specifically heterogeneous sample and a method that permits one to investigate by means of the device according to the invention the spatial distribution of properties of a sample to be examined in all the three directions of space at a spatial resolution going down to atomic resolution. Microscopic investigation directly occurs in local space, a roundabout via scattering methods is not necessary, so that specifically such samples can be examined that do not have a regular internal structure.
The device according to the invention foots on a novel combination of a microscope used for the three-dimensional detection of the topography zn(x, y) of the surface n of a sample, of a probe that detects one or several properties Pj (j=1, . . . , m) of the specimen resolved in space on the topography zn(x, y) of the surface n, of an ablating device, for example a device for plasma etching, for etching with reactive gases or liquids, or for chemimechanical polishing, that is provided with a control and by means of which, in an ablative process An, n+1, a layer may be removed from the surface n of the sample, and of a computer-assisted image processing device that is fitted to produce a three-dimensional image of the spatial distribution of the properties Pj in the sample from a sequence of surface topographies zn(x, y) to zn+m(x, y) and from the properties Pj(zn(x, y)) to Pj(zn+m(x, y)) detected on these topographies.
The method according to the invention foots on a novel combination of microscopy techniques, more specifically of scanning probe microscopy, that attain a very high lateral resolution, with appropriate ablative methods that act globally or locally onto the sample and permit purposeful ablation of layers from the surface of the sample, the thickness of the ablated layer ranging from the atomic range to considerably more. The spatial structure of a sample to be examined is investigated by means of a sequence of individual steps. The topography zn(x, y) of the surface n of the sample to be examined is determined by means of methods of scanning probe microscopy with very high spatial resolution. Additionally, the properties Pj of interest of the sample are locally detected on the surface n, the topography zn(x, y) of which is known, by means of a probe. Examples of such properties are hardness, elasticity, coefficient of friction, conductivity, magnetization, or density of electrons, which often can be detected simultaneously with the determination of the topography, in particular when a scanning probe microscopy technique is made use of. A three-dimensional map Sn of the surface n is built from the topography zn(x, y) of the surfaces n and from the properties Pj(x, y) locally detected thereon and the locally determined properties of the sample are recorded on said map. Accordingly, the map Sn reproduces the topography zn(x, y) with the properties Pj(x, y) recorded thereon, thus representing Pj(zn).
It has to be noted in particular that the surface n of the sample is not assumed to be a plane area, the factual topography zn(x, y) is rather taken as a basis for the three-dimensional map Sn built. Therefore, each single map Sn will generally represent a curved area in space.
Then, a layer of the surface of the sample is globally or locally removed by means of an appropriate ablative procedure. Depending upon the sample to be examined, the ablative procedures that are available may for example be etching with reactive gases and fluids, etching with ions (plasma etching) or chemimechanical polishing. The thus generated new surface n+1 of the sample to be examined is again characterized by means of scanning probe microscopy techniques, i.e., its topography is determined and the properties of interest of the specimen are locally detected thereon. Again, a three-dimensional map Sn+1 of the surface n+1 generated by the ablating procedure An, n+1 is built from the topography zn(x, y) of the surface n+1 and from the properties Pj locally detected thereon and the locally determined properties Pj of the sample are recorded on said map, i.e., a new three-dimensional map Sn+1 of the local properties Pj of the sample is created.
An image of the spatial distribution of the properties Pj in the sample is created by successively ablating further layers and by subsequently characterizing the surfaces obtained. From the xe2x80x9cheapxe2x80x9d of the maps Sk obtained therefrom and which represent the respective surfaces k after each ablation with the spatially resolved determined properties Pj of the sample, it is possible to build up a three-dimensional image of the investigated properties of the sample. By means of a sequence of successive ablating and characterizing steps, it is thus possible to gradually determine the spatial structure, in particular the spatial distribution of the properties Pj in the sample to be examined.
The thereby achieved lateral resolution (in the x-y plane) is limited by the lateral resolution of the methods of microscopy, more specifically of the methods of scanning probe microscopy, that is, it may be in the atomic range. The topography of the created new surface is determined in addition to the spatially resolved detection of the properties of interest, the resolution in z-direction being hereby only limited by the resolution in z-direction of the method of microscopy, more specifically of the method of scanning probe microscopy, thus being readily capable of achieving atomic resolution in this direction as well.
Accordingly, resolution in depth of the method of microscopy according to the invention is substantially determined by the average spacing ak, k+1 of consecutive surfaces which are laid bare in the course of the individual ablative procedures Ak, k+1 and are characterized by means of methods of scanning probe microscopy.
Depending on the kind of sample that is to be examined, a plurality of different ablative methods are available that permit controlled ablation of layers of thicknesses ranging from many nanometers to fractions of nanometers. Accordingly, a combination of methods of microscopy intended to be used in detecting the topography of sample surfaces in three dimensions in space, more specifically methods of scanning probe microscopy combined with such ablation methods, permits lateral resolution and resolution in depth as well, whereby resolution may be in the atomic range (i.e. in the range of 100 picometers).
On the other end of the scale, structures or distributions of properties Pj in a sample may also be studied on a scale of magnitude of micrometers and above by means of appropriate methods of microscopy. The methods of choice are in particular methods of optical microscopy and methods of scanning microscopy that are particularly devised for this purpose.
Resolution in depth (in z-direction) of the method according to the invention, as contrasted with the ablative methods of microscopy mentioned in and known from the state of the art, is not limited by the roughness r of the surface of the sample. Resolution in depth of the method according to the invention is even achieved totally irrespective of the roughness of the surface of the sample, thus not being restricted to application on substantially even, smooth surfaces as it is the case with the previously known methods.
The current teaching foots on the existence of substantially even, smooth surfaces as a basic condition for achieving a high resolution in depth. On using the method according to the invention, as contrasted to the current teaching, a resolution in depth lying within the range of sub nanometers may be achieved over the entire depth to be examined of the sample even on very uneven surfaces. In complete contrast to the current teaching, the method according to the invention also permits one to investigate single objects that have a quite strongly curved surface in all three directions in space with high spatial resolution.
Applicability of the method according to the invention to very uneven surfaces more specifically signifies that the method can be used on very heterogeneous samples in which the quantity of ablated material may strongly vary locally on account of the heterogeneity of the sample. It may as well be used on samples for which no method of uniform ablation is known. According to the current teaching, such samples could not be examined with a high resolution in depth by successive ablation and subsequent characterization of the surface, but they are fully suited to the method according to the invention, even with a high resolution in depth.
Another advantage is that a local rate of ablation of the material may be gathered from the determined data. This rate of ablation will generally be specific to the material. It may accordingly be used as a novel contrast mechanism on creating two- or three-dimensional images of the sample, said mechanism permitting one in particular to distinguish between various materials in the sample.
Furthermore, the device according to the invention and the method according to the invention are suited for complete automation. For the all-automatic characterization of a surface by means of methods of microscopy, more specifically of methods of scanning probe microscopy, a plurality of commercial instruments are available. The automatic ablation of layers; from the surface of a sample by means of ablation methods is also state of the art. Many methods of scanning probe microscopy achieve a lateral resolution that is comparable to and even better than the one achieved with scanning electron microscopy. Therefore, the method according to the invention permits one to investigate samples, which could hitherto only be examined with difficulty at high spatial resolution with the method of the serial sections and electron microscopy, at a comparable and in many cases even better spatial resolution.
Full automation of the device according to the invention and of the method according to the invention makes it possible to determine the three-dimensional structure and properties of a sample with a considerably reduced need for staff and thus at considerably reduced cost than hitherto, which renders it suited for broad industrial application.
The fact that, unlike the scanning electron microscopy or the dynamic SIMS, many methods of microscopy, more specifically of scanning probe microscopy, do not require a vacuum, contributes in rendering the method according to the invention less expensive than known methods operating in space for examining samples with irregular spatial structure and high spatial resolution.
First and foremost however, the device according to the invention and the method according to the invention allows the three-dimensional spatial detection of properties of a sample at a high spatial resolution in all three directions in space which could heretofore only be detected at high lateral resolution on the surface of the sample with the hitherto known methods of scanning probe microscopy. As a result thereof, these methods of scanning probe microscopy are extended to the third direction in space while the high spatial resolution of the scanning probe microscopy is maintained. Many of these properties as, e.g. hardness, elasticity, magnetization, conductivity, could heretofore not be spatially detected at all, or only with a considerably worse resolution, and it is only thanks to the method according to the invention that they may be spatially detected at a high spatial resolution.
The device according to the invention and the method according to the invention of detecting properties of interest of a sample in the three dimensions in space find their application in many fields of science and technique. Examples of their application are:
Detection of the structure of materials consisting of any kind of alloys (metallic, ceramic, or polymeric materials) and of composite materials.
Material testing, more specifically identification and measurement of fractures and other defects.
Study of the three-dimensional spatial structure of self-organized macromolecules.
Spatial detection of charge carrier and dopant atom concentrations in semiconductors and components of semiconductors at a hitherto unattainable spatial resolution. On principle, the position of individual dopants in the semiconductor can be detected with atomic accuracy.
Study of the spatial structure of components of semiconductors that generally consist of a sophisticated spatial array of various materials in very little space.
Investigation of biologic samples, e.g. ultrastructures of cells, viruses, and so on. If need be, dyeing may hereby be relinquished and properties of the samples may be examined that could not heretofore be detected spatially.